Instead, Castle was left to tough out his last weeks of recuperation from a broken kneecap, while Beckett & Co. delved into the sketchy caught-on-camera murder of an IRS investigator. Equipped with binoculars gifted to him by daughter Alexis — and reeled in by a none-too-subtle cue from Esposito — Castle soon becomes fixated on the goings-on of a couple in a neighboring apartment building, where the guy (seemingly) caught wind of his gal’s infidelity and (seemingly) knifed her to death in the bedroom.

Coming up well shy of any smoking gun/Ginsu to support his theory, Castle has a tough time getting Kate on board — especially after Ryan and Esposito (seemingly) question the guy in the apartment. Plus, the detectives claim to have spoken on the phone with the murder victim, who merely had gone off to her mother’s. Castle nonetheless smells something fishy, later witnessing his subject don a gas mask and lug bleach into the (unseen) bedroom, and carry away a rolled-up carpet.

Enlisting Alexis for his endeavor, Castle hobbles next door and sneaks into the neighbor’s apartment to retrieve the shredded contents of the “dead” girlfriend’s purse. Instead, all Rick comes away with is a receipt for a storage locker — in which he and a “reluctantly” dragged-along Kate find… nothing. Nothing but a subsequent talking-to from Captain Gates, after they got pinched by a security guard.

Even so, Rick is resolved to drawing out this cold-blooded killer, after deducing that the chopped-up ex was stashed in the fridge. Desperate to put the issue to rest — if only so that they might get on with Rick’s birthday dinner! — Kate pays the guy a visit. Rick watches as Kate receives a less-than-warm welcome once she makes tracks for the refrigerator. Seeing a melee ensue, Rick calls in the troops and they all storm the apartment… where a surprise party awaits the birthday boy.

Yep, Kate — appreciating how cooped-up and bored and hapless Rick felt — had arranged for this elaborate Rear Window-themed bit of entertainment, with help from some of Martha’s acting students. (Having been slightly spoiled for the episode, it was easy to figure out Martha’s involvement.) Rick’s reaction to being roped in by the ruse: “This is… without a doubt… the greatest birthday gift of my life!”

Later, after the invalid unwittingly helps Kate crack her real case, Rick expresses again how touched he was by the gesture. “What you did for me…. Nobody’s ever done that for me before. That was… epic.” He then warns, “But you do know I will now have to get you back.”

“Oh, you think you can top that?”

“Just you wait and see,” he teases, as he carries her — via wheelchair — into the bedroom.